1. Field of the Invention
The present invention relates to a toner and a developer used in electrophotographic image forming apparatuses such as copiers, facsimiles and printers; and to an image forming apparatus and an image forming method using them.
2. Description of the Related Art
In an electrophotographic image forming apparatus or electrostatic recording device, an electric or magnetic latent image is developed into a toner image. For example, in electrophotography, an electrostatic latent image is formed on a photoreceptor and is developed into a toner image. The toner image is transferred onto a recording medium, such as paper, and fixed thereon by application of heat, etc.
Japanese Patent No. JP-2909873-B1 (Japanese published unexamined application No. JP-H07-120975-A) describes a toner including a polylactic acid as a binder resin. Polylactic acids, derived from plant resources, are widely used and easily available. Japanese Patent Nos. JP-3347406-B1 (Japanese published unexamined application No. JP-H07-033861-A) and Japanese published unexamined application No. JP-S59-096123-A describe that polylactic acid is obtainable by dehydration condensation of lactic acid monomer or ring-opening polymerization of cyclic lactide of lactic acid. Polylactic acid generally includes a larger content of ester groups than polyester resin. Ester group consists of carbon atoms only. It may be difficult to adjust toner properties with polylactic acids only.
Attempts to use polylactic acid in combination with another resin or to copolymerize polylactic acid with another resin have been made. Japanese Patent No. JP-3785011-B1 (Japanese published unexamined application No. JP-2001-166537-A) describes a toner including a biodegradable polylactic acid-based biodegradable resin in combination with a terpene phenol copolymer. Polylactic acids are poorly compatible with or dispersible in polyester resins or styrene-acrylic copolymers that are widely used as binder resins. This may be disadvantageous in terms of controllability of toner surface composition that has an influence on toner properties such as storageability, chargeability, and fluidity.
Japanese Patent Application Publication No. JP-2008-262179-A describes a toner including a block copolymer resin of a polyester having a polylactic acid backbone having a specific D/L ratio with another polyester, in combination with another resin. However, the binder resin using the polylactic acid does not always have high toughness, and background fouling and toner scattering occur due to low toughness when stirred for long periods.
Typically, the binder resin used in a toner is required to have toughness besides chargeability and fixability. When a resin having insufficient toughness is used in a toner, the toner cracks or lacks due to contact stress. A lacked toner is likely to expose an inner wax component having a low melting point, and electrostatically or non-electrostatically remains on a carrier, resulting in toner filming. The carrier contaminated thereby deteriorates in chargeability, resulting in background fouling, i.e., printed toner on a blank part. Similarly, a charge quantity a toner can obtain from a carrier decreases and capability of electrostatically retaining a toner on the surface of a carrier deteriorates, resulting in known toner scattering in apparatus. Even the binder resin using the polylactic acid does not satisfactorily improve durability of the toner against stress when stirred for long periods. Further, in terms of energy saving, reduction of an energy required to fix a toner image is being more demanded.
Because of these reasons, a need exist for a toner using a polylactic backbone as a binder resin and having lower fixable minimum temperature without being solidified when stored for long periods, background fouling, filming and scattering.